The OpenCL ICD extension allows for "Installable Client Drivers" so that multiple OpenCL implementations can co-exist on the same system and be installed at the same time. In turn the OpenCL ICD loader library handles the responsibility of letting CL-using applications choose which platform/device they wish.
OpenCL ICD makes having multiple GPUs/drivers easier or also CPU-based OpenCL implementations. Driver packaging is also made simpler. The original Mesa OpenCL ICD support was published this past summer but only today the support has been committed to mainline Mesa ahead of the Mesa 10.0 release in November.
Adding the CL ICD extension support to Mesa happened with this Git commit and there were also other other Clover Git commits too for laying the groundwork to this new support.
More details on the current OpenCL Gallium3D/Mesa support can be found via the Clover presentation from last month.